1. Technical Field
The present invention relates to a light source device and a projector.
2. Related Art
In recent years, for example, the brightness of projectors has been increasing and the size of projectors has been decreasing annually, and a high output supervoltage mercury lamp that generates strong ultraviolet rays is being used as a light source in such projectors. Consequently, due to mainly ultraviolet light, degradation occurs in components composed of organic matter such as liquid crystal panels, polarizing plates, and wave plates and the like which are used for internal optical systems, and display quality becomes degraded in a short time.
In projectors, in order to protect components such as the liquid crystal panel from ultraviolet light generated by the light source, it is necessary to have an ultraviolet ray cut filter (herein below, called an ultraviolet cut filter) arranged in the light path between the light source and the liquid crystal panel. An example of such an ultraviolet cut filter is one constituted by a dielectric multilayer film formed by using a vacuum deposition method.
However, in the case where an ultraviolet cut filter formed of a dielectric multilayer film described above is used in a projector, it is necessary for the filter to have spectral characteristics that sufficiently cut ultraviolet rays while securing transmittance in the transmission band. In order to attain these spectral characteristics, it is necessary to stack, for example, 40 or more dielectric layers. However, in such a case, it is difficult to maintain necessary film thickness precision from the start to the end of the film formation of the dielectric film, and because of the deterioration of productivity as a result of the film formation time becoming extremely long, practical use is low.
In contrast, there is a known technology (for example, JP-A-2007-212733) in which an ultraviolet cut filter which can be formed by stable film formation (for example, a stacking layer number of 20 layers or less) and an ultraviolet cut filter of the ultraviolet absorption type are used in such a manner as to secure necessary spectral characteristics.
However, in ultraviolet cut filters, it is desirable for the cut-off characteristic that changes from the cut wavelength to the transmission wavelength to be steep. This is because, in the case where the cut-off characteristic is steep, the boundary between the cut wavelength and the transmission wavelength is distinct; therefore by transmitting light having only a particular wavelength component, such light is useable as image light.
If strong light is incident on an ultraviolet-absorption-type ultraviolet cut filter, there is a problem in that heat is generated and the transmittance near the cut wavelength does not have a steep characteristic. Consequently, it is conceivable to stack a plurality of ultraviolet cut filters each formed of a light-reflection-type dielectric multilayer film. However, there is a concern that, in the case where dielectric multilayer films are stacked in such a manner, because a sufficient cut-off characteristic (steepness) is not obtained, the boundary between the cut wavelength and the transmission wavelength is not distinct, and an image light including a component having a wavelength other than a certain wavelength is transmitted thereby causing a decrease in image quality.